Refrigeration systems that use dichlorodifluoromethane (i.e., CFC-12) as the refrigerant generally use mineral oils to lubricate the compressor. (See for example the discussion in Chapter 32 of the 1980 ASHRAE Systems Handbook.) CFC-12 is completely miscible with such oils throughout the entire range of refrigeration system temperatures, i.e., -45.degree. C. to 65.degree. C. In automotive air-conditioning, paraffinic and naphthenic oils of about 500 SUS viscosity at 100.degree. F. are usually used with CFC-12. These oils have "pour points" below -20.degree. C. and viscosities of about 55 SUS at 210.degree. F. and are completely miscible with the CFC-12 refrigerant over the range of temperatures from -10.degree. C. to 100.degree. C. Consequently, oil which dissolves in the refrigerant travels through the refrigeration loop in the air conditioning system and returns with the refrigerant to the compressor. It does not separate during condensation, although it may accumulate because of the low temperature when the refrigerant is evaporated. At the same time, this oil which lubricates the compressor will contain some refrigerant which, in turn, may affect its lubricating properties.
If lubricant separates from refrigerant during operation of the system, serious problems may result, i.e., the compressor could be inadequately lubricated. This would be most serious in automotive air-conditioning systems because the compressors are not separately lubricated and a mixture of refrigerant and lubricant circulate throughout the entire system. Two publications of ASHRAE Transactions, Vol. 90, 2B (1984) discuss the problems associated with separation of lubricants and refrigerants. These are H. H. Kruse et al., "Fundamentals of Lubrication in Refrigerating Systems and Heat Pumps" at pages 763-782, and H. O. Spauschus, "Evaluation of Lubricants for Refrigeration and Air-Conditioning Compressors" at pages 784-798.
Refrigerants which are not completely miscible with an oil in the full range of mixture compositions and operating temperatures may become miscible or immiscible as the temperature is raised or lowered from room temperature. The areas of immiscibility may assume a variety of shapes, i.e., parabolic or non-parabolic. As a parabola, the curve of miscibility temperature vs. percent oil in the mixture, may have its open or concave portion facing the low or high temperatures. The closed or convex portion of the parabolic curve identifies, respectively, the maximum or minimum temperature above or below which the refrigerant and the lubricating oil are completely miscible. These temperatures are referred to as the maximum or minimum "consolute temperatures." Beside parabolas, these curves can assume skewed parabolic shapes or curves of varying slope wherein immiscibility occurs above or below the curve.
The compounds 1,1,1,2-tetrafluoroethane (i.e., HFC-134a), and 1,1,2,2-tetrafluoroethane (i.e., HFC-134), etc. and to a lesser extent, pentafluoroethane (i.e., HFC-125) are being considered as replacements for dichlorodifluoromethane (CFC-12), particularly in automotive air conditioning systems. When substituting a compound such as HFC-134a or HFC-134, or mixtures thereof for CFC-12 in these refrigeration systems, it would be desirable to be able to use the same oils as used with CFC-12. It would not require any substantial change in equipment nor any significant changes in conditions used for the system. Unfortunately, the mineral oils are substantially immiscible with the tetrafluoroethanes.
Lubricants employing polyoxyalkylene glycols have been considered for use with CFC-12 and its replacements. These lubricants may be classified on the basis of their functionality with respect to hydroxyl groups. U.S. Pat. No. 4,755,316, issued Jul. 5, 1988, to Allied-Signal Inc. relates to the use of polyoxyalkylene glycols which are at least difunctional with respect to hydroxyl groups. Research Disclosure 17463 entitled "Refrigeration Oil" by E. I. du Pont de Nemours and Company discloses polyalkylene glycols such as Ucon.RTM. LB-165 and Ucon.RTM. LB-525 sold by Union Carbide Corporation, for use with HFC-134a. These glycols are polyoxypropylene glycols that are monofunctional. Research Disclosure 17463 indicates that the combinations of oil and refrigerant are considered miscible in all proportions at temperatures at least as low as -50.degree. C. and are thermally stable in the presence of steel, copper and aluminum at 175.degree. C. for about six days. However, U.S. Pat. No. 4,755,316, indicates at column 5, lines 14-19- that Ucon.RTM. LB-525 has been found to be immiscible at above about +5.degree. C. with HFC-134a, and that, practically, this means that such lubricants are not miscible with HFC-134a over most of the temperature range used in automotive air conditioning (generally from -40.degree. C. to at least +50.degree. C. or higher, and above 90.degree. C., if possible). U.S. Pat. No. 5,053,155 discusses the use of compositions comprising lubricants containing certain monofunctional or certain difunctional materials.
U.S. Pat. No. 4,975,212 relates to certain novel lubricating compositions comprising a polyoxyalkylene glycol having a cap of a fluorinated alkyl group on at least one end thereof. However, hydroxyl group functionality of the polyoxyalkylene glycol is reduced by such capping.